Titanium has many desirable properties that make it a suitable material for a variety of applications. For example, titanium has a relatively high specific strength, high corrosion resistance, favorable performance characteristics at elevated temperatures, and relatively high bio-compatibility. Such properties make titanium a suitable material for aerospace applications such as for use in turbine and rocket engines and in the medical field such as for prosthetic devices.
Unfortunately, the cost of producing titanium articles from solid stock such as from titanium forgings or from titanium plate is relatively high due to the relatively high cost of titanium stock and the high cost of forming the titanium stock into the desired shape. Furthermore, machining titanium articles from solid stock results in a significant amount of waste material. In addition, titanium has a relatively high hardness which complicates the machining process.
The high cost of producing titanium articles from solid stock has lead to increased development in powder metallurgy. One of the advantages of using powder metallurgy is that articles can be produced at near-net shape which significantly reduces the amount of machining required and reduces the amount of waste material generated. In addition, the use of powder metallurgy to form articles may result in improved mechanical properties in such articles. For example, titanium articles that are formed using powder metallurgy may have a more uniform microstructure and a more homogeneous composition relative to titanium articles produced using conventional ingot metallurgy.
Although powder metallurgy reduces the cost of producing titanium articles compared to conventional production techniques such as machining, the cost of producing titanium articles using powder metallurgy is still relatively high compared to the cost of producing articles from other materials such as from aluminum or alloy steel. Several processes have been developed to lower the cost of producing titanium powder for use in powder metallurgy. Such processes rely on chemical synthesis and are referred to as low-cost direct reduction processes for producing titanium powder. For example, the Armstrong process is a technique wherein relatively high purity titanium powder is produced by injecting titanium tetrachloride vapor into a stream of molten sodium. The sodium cools and the reaction products—titanium, sodium, and salt—are separated. The process results in a continuous stream of titanium powder suitable for use in powder metallurgy for forming titanium articles.
Although relatively low in cost compared to titanium powder produced using conventional techniques, titanium powder produced by the Armstrong process results in individual powder particles having a relatively low individual density. In addition, titanium powder produced by the Armstrong process has a low bulk density relative to the true or theoretical density of titanium. The bulk density may be described as the tapped density of loose powder particles in a container prior to compaction of the powder into a green structure and prior to consolidation of the green structure into the final article. The theoretical density of a powder is the density of the powder if melted into a solid mass. The bulk density of a powder may be dependent upon several factors such as the shape of individual powder particles and the cohesiveness between the particles, both of which affect the ability of the powder particles to move closer to one another and reduce the bulk density. In the case of powder produced by the Armstrong and other chemical synthesis processes, the bulk density of such powder is typically less than approximately 10 percent of theoretical density.
Unfortunately, in order to achieve a relatively high density in the final article, many powder metallurgy processes may require a bulk density that is higher than the bulk density of powder produced by the Armstrong process. For example, certain power metallurgy processes require a bulk density that is no less than approximately 50 percent of theoretical density in order to achieve the necessary density in the final article. A relatively high density in the final article is desirable because the mechanical properties such as strength and fatigue resistance of the article are typically directly related to the density of the article.
As can be seen, there exists a need in the art for a system of method for increasing the bulk density of relatively low-density metal powders for use in powder metallurgy.